Crystal Structure and Physical Properties of the Lanthanum Chalcoantimonate TlLa2Sb3Se9

The new lanthanum chalcoantimonate TlLa2Sb3Se9 has been synthesized and its crystal structure determined. TlLa2Sb3Se9 crystallizes in an ordered variant of the KLa2Sb3S9 type, space group P2(1)2(1)2(1) with the lattice parameters a=4.2621(2) angstrom, b=15.155(5) angstrom, c=25.505(9) angstrom. The band gap of TlLa2Sb3Se9 was calculated to be 0.47 eV, and experimentally determined to be 0.68 eV. Its thermoelectric properties were optimized via doping with Ca2+; samples with the compositions TlLa2-xCaxSb3Se9 (x=0.01, 0.03, 0.05) were synthesized. Despite ultralow thermal conductivity, the maximum thermoelectric figure-of-merit of the undoped sample was only zT=0.031 at 623 K, which was increased to 0.078 for the sample with the nominal composition of TlLa1.95Ca0.05Sb3Se9. These low values are a consequence of the uncompetitively low electrical conductivity.